A portable power tool 100 shown in FIG. 1 includes a table 102 with a work surface 104 for supporting a workpiece. The table defines a slot 109 through which a cutting tool 108, such as a rotary saw blade, extends. The cutting tool is typically powered by a drive motor (not shown) and is adjustably supported by apparatus 110 beneath the work surface of the table 102. For instance, the apparatus 110 may include components for adjusting the height or angle of the cutting tool 108 relative to the work surface. The components may be manual, such as a hand crank and gearing, or may be motor-driven. The apparatus 110 will also include one or more activation switches to control the operation of the various components of the power tool 100, including a mains switch 112 that controls the drive motor for the tool.
Power tools require various safety devices or mechanisms to prevent injury to the tool operator. Some safety devices are integrated into the control system, such as automatic shut-off features. Other safety devices are mechanical, in the nature of shields and guards that prevent the operator from accidentally coming into contact with an operating tool, such as a rotary saw blade 108 rotating at no-load or operating speeds. One such mechanical safety device is a riving knife 106 that is positioned within the downstream end of the slot 109 to reduce the likelihood of a kick-back event in which the workpiece gets caught or bound up during a cutting operation. As shown in FIG. 2, the riving knife includes a mounting slot 120 that allows the riving knife to be retracted beneath the work surface 104. A clamping mechanism 122 (see also FIG. 3) engages the slot 120 of the riving knife and allows the operator to raise or lower the knife to position it as desired, such as when performing a partial cut in the workpiece. The clamping mechanism 122 is supported on a carriage 124 that forms part of the apparatus 110 associated with the tool 108. One example of a riving knife arrangement is disclosed in U.S. Pat. No. 8,127,648, which issued on Mar. 6, 2012.
As seen in FIGS. 1, 2, the riving knife protects the downstream end of the tool or saw blade 108. Even when the riving knife is fully extended, an upstream portion of the blade 108 is still exposed. Consequently, many safety systems for power tools include an upper guard, such as the upper guard 132 shown in FIGS. 3, 4. The upper guard includes a lower edge 135 configured to contact the surface of the workpiece continuously as the cut is being performed. For optimum safety the riving knife and upper guard would be permanently mounted to the power tool 100. However, the reality of decades of historical use of table saws is that commercial artisans as well as experienced woodworkers want to and do use table saws to make specialty cuts, including non-through cuts, plunge cuts, cove cuts and dado cuts, for example. A plunge cut is made by placing a workpiece on the saw with the blade retracted, turning on the motor and cranking the blade upwardly to make a cut more or less to the middle of the work piece. A dado cut and a cover cut are other specialty non-through cuts utilizing special tools. Consequently, the upper guard 132 is typically removably supported on a mounting portion 130 of the riving knife by a mounting mechanism 134 that allows the upper guard to pivot when mounted to the riving knife. One example of a removably mounted upper guard is shown in U.S. Pat. No. 8,096,220, which issued on Jan. 17, 2012.
While it may be desirable for the upper guard to be removed for specialty or non-through cuts, for through cuts it is important to have the riving knife 106 and upper guard 132 in proper position protecting the operator from the working blade 108. It is therefore desirable to integrate a safety apparatus into the tool 100 that helps prevent usage of the power tool without the safety mechanism properly in place.